Previously we observed that active immunity to RS virus is of a different nature in the lungs than the nose, pulmonary immunity being more durable. The nature of active immunity was then studied using the technique of parabiosis. Parabiosis stduies in cotton rats established that there is a systemic component of active immunity to RS virus. This systemic component provided resistance in the lungs but not the nose. Preliminary evidence suggests that serum antibodies play a role in this resistance. Maternally-transmitted immunity to RS virus was also studied in cotton rats because a recent epidemiologic study of infants and children suggested that breast feeding conferred demonstrable resistance to serious RS virus respiratory tract disease during the first two years of life. We observed that passive immunity in cotton rats was transferred from the mother to her infants. This form of immunity was transient, lasting one week in the nose and 4 weeks in the lungs. Most of this type of immunity was transferred during the post natal period via colostrum and milk.